The invention relates to a device for precisely clamping flat objects without play. Clamping devices of this type are used for clamping printing plates on a plate cylinder of a printing machine, for example.
Such a clamping device should enable a precise clamping which remains immobile even when forces act upon the object that is clamped in. A problem that arises in the production of a precise clamping is that it is very difficult to produce exactly parallel clamping surfaces on both sides of a clamping gap over longer distances. The published German Patent Document DE 43 00 099 C1 proposes that this objective be achieved by a clamping device having two clamping surfaces which are disposed on either side of a gap and which are displaceable relative to one another along the width thereof, one of the clamping surfaces being formed by lateral surfaces of a plurality of plate-type spring elements. These spring elements are arranged in succession along the length of the gap and are rotatable to a defined extent in order to achieve parallelism through self-alignment during the tightening or tensioning of the clamping surfaces. But the problem may arise herein that the distribution of the clamping force, though uniform over the length of an individual spring element, varies over the overall length of the gap due to size deviations among the various spring elements. Thus, during the operation of a printing plate which has been clamped in a clamping device of this type, the printing plate may wander several hundredths of a millimeter before finding a final position. Because no prediction can be made when clamping a printing plate as to whether and in what direction such wandering will occur, the wandering naturally interferes greatly with the print results obtained by using the plate.
It is accordingly an object of the invention to provide a clamping device for flat objects which ensures uniform clamping over the longitudinal extent of a gap and which prevents wandering.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a clamping device having clamping surfaces disposed on both sides of a gap and being movable relative to one another, at least one of the clamping surfaces being formed by lateral surfaces of a plurality of plate-type spring elements, comprising, at the lateral surface of each spring element, local projections facing the gap and separated by returns, the spring elements being arranged in a stack so that a projection of a spring element is adjoined by respective returns of adjacent spring elements, each of the projections being disposed over a recess formed in the respective spring element.
In accordance with another feature of the invention, the respective projection is disposed centrally over the recess and is carried by arms at opposite sides thereof.
In accordance with a further feature of the invention, mutually adjacent ones of the recesses are separated by a bridge having a longitudinal extent that is at the most half that of the recesses.
In accordance with an added feature of the invention, each of the spring elements has a lateral surface facing away from the gap and formed with a projection located across from the respective projection of the lateral surface facing the gap.
In accordance with an additional feature of the invention, the spring elements arranged in the stack are in staggered formation, so that an end of a respective projection of one of the spring elements is adjoined in the longitudinal direction by a beginning of a respective projection of another of the spring elements.
In accordance with one alternative feature of the invention, the projections of a plurality of the spring elements form a spar pattern.
In accordance with another alternative feature of the invention, the projections of a plurality of the spring elements form a pattern symmetrical to a central plane of the stack.
In accordance with yet another feature of the invention, an odd number of spring elements is provided.
In accordance with yet a further feature of the invention, the length of the projections is at least 2/(nxe2x88x921) times the length of the returns, and preferably 2/(nxe2x88x923) times the length, wherein n is the number of spring elements.
In accordance with an added feature of the invention, the spring elements are formed of steel, and particularly of a chrome-nickel steel.
In accordance with an additional feature of the invention, the spring elements have a thickness of from 0.3 to 1 mm, and the projections have a length of from 5 to 20 mm.
In accordance with a concomitant aspect of the invention, there is provided a printing cylinder for a printing machine, comprising a clamping device for clamping a printing plate, having at least one of the foregoing features.
Preferably, each projection of a respective spring element of this type is disposed over a recess which enables the projection to yield resiliently or elastically in the direction of the recess under an exerted pressure. The projection is preferably disposed over the recess centrally and carried at two sides by arms.
Adjacent recesses of a spring element are preferably separated by a bridge having a longitudinal extent that is at most half as great as that of the recess. This makes it possible to hold the stack of spring elements together by pins or screws extending through the recesses and potentially through a borehole formed in the bridge.
According to a preferred development, the spring elements have bridges not only at the lateral surface facing the gap, but also at an opposite lateral surface which faces away from the gap. It is therefore possible to halve the spring constants of the spacing elements while maintaining the same material thickness, and thus to compensate for larger deviations from the parallelism of the clamping surfaces than would be possible using a spring element that is elastic only on one side.
In order to achieve an optimally uniform hold for the clamped object, the spring elements of the stack are preferably arranged in a staggered pattern, so that an end of a projection is adjoined by the beginning of a projection of another spring element in the longitudinal direction.
This type of arrangement can be an arrangement of projections in a diagonally striped pattern, for example. A particularly preferred arrangement is one wherein the diagonal stripes form a spar pattern, because forces which could effect a shifting of the clamped object along the length of the gap largely compensate for one another, if such a pattern is provided.
For the same reason, a pattern of projections which is symmetrical with respect to a central plane of the stack, the central plane extending parallel to the longitudinal direction, is generally preferred.
This type of pattern can be realized economically by employing an odd number of spring elements. In this case, the central plane extends through the central spring element of such a stack.
To ensure that, over the length of the gap, at least one projection of a symmetrical pattern is always in contact with the clamped object, the length of the projections must equal at least 2/(nxe2x88x921) times the length of the returns, where n is the number of spring elements. Assuming a spar pattern, in order to ensure an overlapping of the spars on the width of at least one projection, the length should rather equal 2/(nxe2x88x923) times.
The spring elements can advantageously be formed of steel, particularly a nickel-chromium steel. Advantageous dimensions, particularly assuming the use of the clamping device in a printing cylinder of a printing machine, are a thickness of 0.3 to 1 mm of the spring elements, and a length of the projections of 5 to 20 mm.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a clamping device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.